2018 H2 Chem P1 Q5 — Mole Solution

The answer is C.
Relative atomic mass compares the average mass of one atom of the element (averaged over all its isotopes, weighted by abundance) with \(\frac{1}{12}\) of the mass of one atom of carbon-12:
\(A_\text{r}(\text{O}) = \dfrac{\text{average mass of one atom of oxygen}}{\frac{1}{12} \times \text{mass of one atom of carbon-12}}\), which is exactly the expression in C.
• A is wrong: a compound is made of molecules (or formula units), not single atoms. One mole of a compound contains the same number of molecules (formula units) as there are atoms in 12.000 g of carbon-12.
• B is wrong: relative isotopic mass refers to one particular isotope, so the top line must be the mass of one atom of lithium-7. The average mass over all the isotopes of lithium gives the relative atomic mass of lithium, not the relative isotopic mass of lithium-7.
• D is wrong on two counts: the top line must be the average mass of one molecule of E, not one atom, and the standard is \(\frac{1}{12}\) of the mass of one atom of carbon-12, not one molecule.